Inks which are used in either flexographic printing processes or letterpress printing presses must adhere to any rollers in the ink train, wet the print roller, then must easily and evenly be transferred over to the printed substrate and form a permanent bond thereto.
A letterpress system has a series of rollers to pick up and form a uniform film of ink that is transferred by these rollers to the cylinder carrying the type. A letterpress uses raised types that are mounted on a cylindrical impression roller. The roller in turn is coated with the ink on the raised portions that form the characters and is rotated against the paper or other substrate to be printed. Flexographic presses use an anilox metering roller in conjunction with form rollers or a doctor blade to define a uniform layer of ink that is then transferred to the print cylinder.
Nowadays, flexographic presses and letterpress presses, for newspapers printing, use types made- of photo-sensitive polymeric materials that can be formed directly from a light image. The polymeric cylinder when properly developed presents raised areas that reproduce the image to be printed. This plate is mounted on a cylindrical roll that rotates against an ink cylinder so as to become coated in its raised areas. These areas rotate against a substrate (generally paper) which is backed by a hardened rubber roller.
Letterpress and flexographic systems can be used with either solvent based or aqueous based ink systems. In either case, current practice consists of mixing a solvent or water in the presence of a binder resin such as, for example, nitrocellulose, maleic anhydride, an acrylic copolymer, or various starch derivatives. The pigment is applied over the paper by the printing roller and the binder serves to hold the pigment particles in place. In some instances, the pigment is carbon black directly suspended in a high boiling point aliphatic hydrocarbon solution. This ink has no binder but instead is fixed to the paper by diffusion of the oil into the paper thus leaving a highly blackened layer of carbon black and oil upon the surface of the paper. These oil-based systems suffer from a number of disadvantages including the fact that since the print never chemically bonds to the paper it can easily smudge when handled. Furthermore there is a tendency for the inks to contaminate the various guide rollers in the system and to produce a background.
A water based flexographic ink composition generally comprises a pigment, a polymer resin, a binder or wax, an organic solvent and water. Such compositions vary depending on the characteristics of the raw materials, the quality required, as well as different properties provided by numerous manufacturers. Trends in the last years resulted in a decrease of the content in volatile organic solvent to limit their impact on the environment. Nowadays, water based flexographic inks usually contain between 5 up to 20% of organic solvents. Although they are harmful, they cannot easily be eradicated because they help to reduce the drying time of the ink once it is applied on a surface (paper, cardboard, plastic, etc.). Only few other concrete efforts have been devoted to improve the environmental aspect of flexographic inks. Therefore, they are still composed of poorly or non-biodegradable pigments and polymer resins that generally accumulate in post-treated sludge resulting from the de-inking processes, or are buried in soils when the final product is disposed therein. Biodegradable pigments already exist, but they do not provide better properties than synthetic pigments. Conventional flexographic ink compositions are normally made of acrylic, methacrylic, epoxy or styrene based polymer resins. Actually, acrylic and epoxy resins are present in 90% of the flexographic ink compositions.
U.S. Pat. No. 5,004,664 describes a toner composition comprising semicrystalline polyester resin particles, polyhydroxyalkanoates and copolymers thereof and pigment particles. This application is suitable for electrostatic printing only.
U.S. Pat. No. 5,114,479 describes the use of a fortified latex. suspension as a vehicle to make dye/latex mixtures for gravure and flexographic inks for newsprint and corrugated applications. They used water-soluble polymers based on acrylic and methacrylic acids.
Tajiri et al. report in U.S. Pat. No. 5,120,360 the use of microcapsules containing ink for flexographic applications. The authors describe that encapsulation of the ink ensures a better adhesion and flowability. Resins used for the microencapsulation are composed of methacrylate or acrylate of molecular weight of 3,000 up to 50,000 g/mol. Vanderhoff and Huwart report in U.S. Pat. No. 5,830,927 an ink composition devoid of volatile organic solvent and containing a water-soluble polymer such as polyvinyl pyrrolidone, polyacrylamide, polyacrylic acid, polyvinyl acetate, polyvinyl alcohol and the likes.
Takashi et al. report in Japan Patent No. 8092518 a biodegradable ink composition based on polylactic acid, without the use of any organic halogenous solvent.
Based on the above-listed patents and publications, which are representative of the state of the art, there is still a large place for improvement in producing a flexographic water based ink composition which induces enhanced environmental properties.